Radio receivers, such as pagers, which receive radio signals frequency modulated by digital signals, are designed to be very small and inexpensive, and typically use both frequency determining circuits and bandpass filters. The frequency determining circuits and bandpass filters have characteristics that are determined at the time of manufacture and which vary from part to part. The frequency determining circuits and bandpass filters also have characteristics determined by time and temperature. For example, the harmonic frequency of a crystal has a part to part make tolerance, as well as time and temperature dependent tolerances. As a further example, the peak frequency, the bandwidth, and the shape of the frequency response curve of a bandpass filter have a part to part make tolerance, and some temperature dependent tolerance. The sensitivity of radio receivers using such frequency determining circuits is affected by these characteristics, as well as an offset of the radio signal being received. The sensitivity of the radio receiver is the ability of the radio receiver to accurately recover information when a radio signal being received is weak or distorted. The offset of the radio signal being received is the difference of the carrier frequency of the radio signal being received from an expected value of the carrier frequency of the radio signal. The sensitivity of the radio receiver is optimized when a local oscillator in the radio receiver converts the carrier frequency of the radio signal to an intermediate frequency (IF) which is optimally aligned to one or more IF filters having a cumulative shape and a bandwidth optimized for the characteristics of the radio signal being received.
For a radio receiver using a design approach involving very low cost and minimal parts count, such as a pager which has receive only capability, the design approach has typically resulted in the use of frequency determining components selected to operate together under worst case conditions of all tolerances, including part to part, temperature, and time tolerances, without including automatic optimal alignment circuits in the radio, such as automatic frequency control circuits, which are well known to one of ordinary skill in the art. This approach has worked well, but this approach limits the receive sensitivity performance in comparison, for example, to that which can be achieved in radio receivers which have relatively high speed, high power circuits, such as radios using synthesized local oscillators and digital signal processors, wherein sophisticated automatic frequency and automatic phase control circuits and methods have been used to optimally align the local oscillator. When optimal alignment methods, including automatic frequency control are not used, the sensitivity is less than optimum because the bandwidth of the IF filter must be widened to accommodate the part to part, time, and temperature variations of the filter (or cumulative filters, when more than one are used), the local oscillator reference (typically a crystal circuit), and the expected received carrier offset.
One method of optimizing receive sensitivity used in pagers is to optimize the part to part variations of frequency determining circuits, such as a crystal circuit, by making adjustments to the value of a part, such as a capacitor, in the crystal circuit. For example, a mechanically variable capacitor is adjusted or a capacitor is laser trimmed in a factory procedure to optimize the frequency of the crystal circuit. Alternatively, in a pager having a voltage controlled oscillator, a correction value to optimize the nominal oscillator frequency can be determined and stored in a factory procedure. These procedures typically adjust the nominal (room temperature) frequency of the frequency determining circuits accurately, using a factory standard frequency reference.
Using a factory standard frequency reference procedure corrects for part to part variations of parts, such as crystals and capacitors, in the frequency determining circuits, but does not adjust the frequency determining circuits to compensate for part to part variations in other circuits, such as an intermediate frequency filter. Such adjustment would further optimize the receive sensitivity of the pager. Unfortunately, presently available procedures involve the use of an analog signal such as a pure carrier, and an analog output, such as an IF output signal, which must be measured to determine a relative gain at various frequencies prior in order to perform an adjustment to optimize the receive sensitivity to compensate for part to part variations in an intermediate frequency filter. This method is too slow to achieve the manufacturing costs necessary to attain competitive pricing.
Another method of optimizing receive sensitivity in a radio receiver having a voltage controllable oscillator and a controller is to adjust frequency determining circuits during the use of the radio receiver. This method compensates for time and temperature variations, and can also compensate for the part to part variations when they are not severe. This method has been accomplished in some radio receivers by analog techniques, which require additional circuitry in radio receivers which are digital in nature, such as an alphanumeric pagers. This method has also been accomplished in other, sophisticated, radio receivers such as cellular radios and commercial portable radios. These sophisticated radios have employed digital signal processors in conjunction with microprocessors for phase locking the radio receivers to a received digital signal, thereby optimizing the receive sensitivity of the radio receiver.
Thus, what is needed is a method of optimizing the sensitivity performance in very small and low cost radios which receive radio signals frequency modulated by a digital signal, which method is usable in a factory alignment method of the radios and/or during use of the radios.